Editing How to rebuild a Rochester Quadrajet 4MV carburetor (section)
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==Casting smoothing== Generally speaking, reworking the castings of a carb without any empirical way of measuring changes is like porting a cylinder head without a flow bench. What looks good may in fact be the exact opposite of what the carb needs, and without knowing what is happening as you make changes, the results can be worse than if you had done nothing at all, so if you decide to do anything, do not change the shapes, angles of attack, or any other parameter that was a designed-in feature of the carb. Keep things to a minimum- smoothing obvious flaws and mismatches, and leave the rest alone. If you are a "burr freak", [http://www.cratex.com/rubpoint.htm Cratex points] can be carefully used in a Dremel tool. Using them will provide the very highest finish of any "Dremel-type" points, especially on curved and/or intricate castings. Some come mounted, others can be mounted on a small arbor w/screw (you know the arbors- like used for the small cutting discs). I would avoid using mounted stones altogether- they will leave a rougher than stock finish, and generally are not as precise as I believe is needed for that type of work. There are paper sanding discs (400-600 grit and finer) that are fine enough; backing them with a round piece of plastic cut from a coffee can lid can give enough backing to allow smoothing and polishing, when mounted on a small arbor. ===Primary=== On the primary side, the booster rings and the venturi can be carefully smoothed where there may be a casting mismatch or casting flash or et cetera. If there's a mismatch between the baseplate and main body, that can be taken care of- but this is not generally a problem, nor has the Q-jet historically had any significant amount of casting irregularities. ===Secondary=== There's no point in doing anything (other than for personal satisfaction) to the secondary side casting unless there's some huge gaping mismatch, etc.The fit of the base plate gasket can be more of a flow problem than any casting irregularity; if the gasket intrudes into the throttle bore, removing THAT will help more than anything. Also, everywhere the casting is ground on or polished removes the protective finish from the zinc of the carb body, and that can lead to the dreaded attack of the "white stuff" that's mentioned anytime old carbs are talked about.
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